1. Field of the Invention
The present invention relates to an illuminating device and a liquid crystal display device, and more particularly, to an illuminating device which generates a surface light ray using a light guide plate, and a liquid crystal display device including the illuminating device used as a backlight.
2. Description of the Related Art
A display device is a medium to show information to persons visually and is very important for humans in the today's highly information-oriented society. In particular, the performance of a liquid crystal display device has been remarkably improved in recent years and the device is being employed as a display device equipped in a mobile phone terminal, a display device for use with a personal computer, a display device for a wide-screen television, etc. In general, such a liquid crystal display device includes a liquid crystal display panel and a backlight (illuminating device) which is placed on the rear side of the liquid crystal display panel to emit light to the liquid crystal display panel.
A liquid crystal display device having a backlight (hereinafter simply referred to as a “liquid crystal display device”) displays a picture or an image by independently controlling transmission of light from the backlight for each of pixels in a liquid crystal display panel. At this point, it is preferable that the liquid crystal display panel has a pair of polarizers with a liquid crystal layer interposed therebetween and performs display by controlling a polarization state of light using the liquid crystal layer in that a picture having a high contrast ratio can be obtained with a relatively low driving voltage. Examples of display modes of such a liquid crystal display panel may include a TN (Twisted Nematic) mode, an STN (Super Twisted Nematic) mode, an ECB (Electrical Controlled Birefringence) mode, an IPS (In-Plane Switching) mode, a VA (Vertical Aligned) mode, and other modes known in the art. In any mode, the liquid crystal display panel typically includes a pair of substrates, a liquid crystal layer interposed between the pair of substrates, and a pair of polarizers with the pair of substrates and the liquid crystal layer interposed therebetween and displays a picture or an image by changing a polarization state of light using the liquid crystal layer to control transmission of light for each pixel.
The polarizers used in the liquid crystal display panel have a function to absorb one linearly polarized light component and transmit another linearly polarized light component whose polarization plane is orthogonal to the one linearly polarized light component. Therefore, if light emitted from the backlight to the liquid crystal display panel (hereinafter referred to as “backlight light”) is unpolarized light, the polarizers provided in the liquid crystal display panel absorb at least 50% of the backlight light. That is, in the liquid crystal display device, if the backlight light is unpolarized light, about half of the backlight light is absorbed by the polarizers, i.e., is lost. Accordingly, it is desirable to decrease a percentage of backlight light absorbed by the polarizers provided in the liquid crystal display panel and improve use efficiency of the backlight light for implementation of a liquid crystal display device providing a brighter image and lower power consumption.
In the meantime, a backlight of the liquid crystal display device may include an edge light type backlight system (light guide type), a direct type backlight system (reflector type backlight system), a surface light source type and the like, which may be used depending on use of the liquid crystal display device. Among these types, the edge light type backlight system can achieve a backlight which is thinner, lighter and provides lower power consumption than other types and is widely used for the liquid crystal display devices equipped in a mobile phone terminal. Such an edge light type backlight includes a line or point light source, a light guide plate which expands light from the light source into surface light to be emitted to a liquid crystal display panel, optical sheets such as a prism sheet and a diffusion sheet, a reflection sheet, etc.
In the edge light type backlight (hereinafter abbreviated as “backlight”), the light emitted from the light guide plate generally has the maximum (peak) of luminance or luminous intensity in a direction inclined by 60° to 80° with respect to a perpendicular (normal) direction of a light emission plane of the light guide plate. Accordingly, between the light guide plate and the liquid crystal display panel is interposed a prism sheet which controls a traveling direction of light having maximal luminance or luminous intensity of the light emitted from the light guide plate.
The prism sheet is a transparent optical member, which generally includes a sheet-like base material and a plurality of prisms arranged on the base material. In addition, each of the prisms generally has a triangular section.
It is known that light emitted at an angle providing the maximum luminance or luminous intensity (peak angle) and an angle near the peak angle among the light emitted from the light guide plate has a greater p-polarized component than an s-polarized component. Accordingly, in recent years, there has been proposed a surface light source device (backlight) using a prism sheet which is configured to preferentially lead light having a greater p-polarized component emitted from a light guide plate toward the front direction (for example, see Japanese Patent No. 3299087).
In the surface light source device disclosed in Japanese Patent No. 3299087, each of prism arrays constituting the prism sheet has two inclined surfaces and an inclination angle of an inclined surface farther from the light source is selected to refract light having a greater p-polarized component emitted from the light guide plate toward the front direction. In addition, an inclination angle of an inclined surface relatively closer from the light source is selected to have a range within which light having a greater p-polarized component emitted from the light guide plate is not incident onto the inclined surface. In this case, it is stated in Japanese Patent No. 3299087 that illuminating light from the surface light source device is suitable for backlights of liquid crystal display devices since the illuminating light has a polarization.